Spectroscopic study of 2-, 4- and 5-substituents on pKa values of imidazole heterocycles prone to intramolecular proton-electrons transfer

New Derivatization Methodology of 4-aminobenzoic Acid from its Dietary Supplements: Kinetic Spectrophotometric Methods for Determination

Background:

A new approach describing the validation and development of an easy, new spectrophotometric and kinetic method for identification of para-aminobenzoic acid in dietary supplement has been performed. In this study, para-aminobenzoic acid was derived in a pH-controlled environment, as a new organic compound 4(4-Benzophenylazo)pyrogallol, by incorporating diazotized para-aminobenzoic acid with pyrogallol.

Objective:

The determination of para-aminobenzoic acid was conducted by the fixed time and initial rate techniques. These approaches were based on the reaction of the compound containing paraaminobenzoic acid, 4(4-Benzophenylazo)pyrogallol, with Ag(I) to form colored product with a maximum absorbance at 468nm. Both of these techniques were adopted for constructing the calibration curves and examined for their suitability for the quantitation of para-aminobenzoic acid in dietary supplement.

Methods:

The determination process was established, using initial rate and fixed time kinetic spectrophotometric methods.

Results:

4(4-Benzophenylazo)pyrogallol was characterized using proton-nuclear magnetic resonance, Fourier-transform infrared, differential scanning calorimetry and thermogravimetric thermal methods, gas chromatography–mass techniques, and solvatochromic behavior in solvents with different polarities was also examined.

Conclusion:

For the first time, para-aminobenzoic acid was well determined by incorporating it as an organic solid compound, 4(4-Benzophenylazo)pyrogallol, through coupling pyrogallol with diazotized para-aminobenzoic acid in regulated pH medium, ranging between 5.0 to 6.0. The existence of common excipients in the dietary supplement did not produce any significant interference. F- and ttest data analysis were used for statistical comparison of the suggested techniques with that of reference method, demonstrating excellent agreement with no significant difference in the associated precision and accuracy.

Comparison of experimental and DFT-calculated NMR chemical shifts of 2-amino and 2-hydroxyl substituted phenyl benzimidazoles, benzoxazoles and benzothiazoles in four solvents using the IEF-PCM solvation model

A comparative study of experimental and calculated NMR chemical shifts of six compounds comprising 2-amino and 2-hydroxy phenyl benzoxazoles/benzothiazoles/benzimidazoles in four solvents is reported. The benzimidazoles showed interesting spectral characteristics, which are discussed. The proton and carbon chemical shifts were similar for all solvents. The largest chemical shift deviations were observed in benzene. The chemical shifts were calculated with density functional theory using a suite of four functionals and basis set combinations. The calculated chemical shifts revealed a good match to the experimentally observed values in most of the solvents. The mean absolute error was used as the primary metric. The use of an additional metric is suggested, which is based on the order of chemical shifts. The DP4 probability measures were also used to compare the experimental and calculated chemical shifts for each compound in the four solvents. Copyright © 2015 John Wiley & Sons, Ltd.

Effective targeting of proton transfer at ground and excited states of ortho-(2′-imidazolyl)naphthol constitutional isomers

Besides electronic effects at the excited state, ground and excited state acidities are largely affected by proximity and structural alignment.

Cobalt(ii) catalyzed C(sp)–H bond functionalization of alkynes with phenyl hydrazines: facile access to diaryl 1,2-diketones

A cobalt acetylacetonate catalyzed oxidative diketonation of alkynes via C(sp)–H bond functionalization has been described. Its application to the synthesis of imidazoles has also been demonstrated.

Spectroscopic and quantum chemical investigations of substituent effects on the azo-hydrazone tautomerism and acid-base properties of arylazo pyridone dyes

A series of 5-(4-substituted arylazo)-6-hydroxy-4-methyl-3-cyano-2-pyridone dyes was synthesized and the structure of the dyes was confirmed by UV-Vis, FT-IR, (1)H NMR and (13)C NMR spectroscopic techniques. The azo-hydrazone tautomeric equilibrium was found to depend on the substituents as well as on the acidity and basicity of the media. Ionization constant, pKa, of the dyes was determined by UV-Vis spectroscopy and correlated with the Hammett substituent constants, σp and σI. The interpretation of the effect of different substituent in phenyl ring of arylazo pyridone dyes on their spectroscopic and structural properties was based on quantum chemical calculations performed by the density functional theory (DFT/M06-2X) method. The DFT calculations confirmed the existence of two forms in water solution: hydrazone form in acidic and neutral media and anionic form in basic media. The different contribution of azo and hydrazone canonical forms of anionic form is observed for dyes with electron-donating and dyes with electron-withdrawing groups. The dependence of absorption spectra and determined pKa values to the substituent type seems to be mostly due to azo/hydrazone canonical structure ratio in their anionic form.

Design, preparation, and implementation of an imidazole-based chiral biaryl P,N-ligand for asymmetric catalysis

A new strategy for increasing the barrier to rotation in biaryls has been developed that allows for the incorporation of 5-membered aromatic heterocycles into chiral atropisomers. Using this concept, an imidazole-based biaryl P,N-ligand has been designed and prepared as a single enantiomer. This ligand performs exceptionally well in the enantioselective A(3)-coupling, demonstrating the potential of this new design element.